award booking - .com won't match up segments?

I'm looking to fly LAX-IAH-home.

-The first segment (787) is available in I for 25k saver.
-The second segment is available in YN on an ERJ for 25k standard (no biz class on equipment).

The on-line won't populate a complete trip economy ticket nor will it populate a mixed class ticket at even 50k. It just won't pull those flights for an award, period.

But it has no problem pulling a complete revenue ticket with those flights.

Shouldn't I be able to purchase something on one ticket and at what cost for what class of service?

CS basically couldn't offer an explanation.

Thanks